When drilling for oil and gas it is desirable to be able to steer the drill bit, i.e. to move the drill bit in a chosen direction, so that the drill bit does not have to follow a path determined only by gravity and/or the drilling conditions.
One method of steering a drill bit is to use a downhole motor with a “bent housing”, i.e. a non-linear shaft. A downhole motor is connected to the drill bit and drives the drill bit to rotate with the remainder of the drill string not rotating, one such downhole motor being a mud motor which uses the flow of drilling mud to drive the drill bit. The bent housing allows the drill bit to follow a non-linear path in the direction of the bend in the shaft.
Often, this method and apparatus will be utilised when the desired direction and degree of curvature of the borehole is known. However, to cater for unexpected drilling conditions an operator will usually design the shaft with a greater bend than necessary, so that the desired degree of curvature can be achieved even if the drilling conditions result in the drill bit deviating from a linear path less than was expected. If, however, the drill bit does deviate as much as expected, this will result in the curvature of the borehole exceeding that desired, so that a linear (or more linear) length of borehole needs to be drilled to compensate. The linear (or more linear) length of borehole is drilled by rotating the whole of the drillstring, which continuously changes the direction of the bend in the shaft and cancels out the tendency to curve in one direction.
Unfortunately, drilling a more linear length of borehole in this way has a number of disadvantages, the most significant being the poor quality borehole which results due to the constantly changing drilling direction. Thus, whilst it is desired that the drilled borehole matches the diameter of the drill bit as closely as possible throughout its length (which facilitates the passage of stabilisers and other componentry along the borehole, and enables more reliable measurements to be obtained from sensors engaging the borehole) this is not always achievable with this method and apparatus.
Other steering apparatus and methods are known, for example the steering component described in our published European patent application EP-A-1025245. That steering component allows the drill bit to be moved in any chosen direction, i.e. the direction (and degree) of curvature of the borehole can be determined during the drilling operation, and as a result of the measured drilling conditions at a particular borehole depth. Despite the advantages of this steering component, some operators require a less complex steering system for the many applications in which the required borehole curvature can be predetermined.